I've been looking through the service manual for this amp, but just cant seem to pin-point where the audio signal is not getting to. This is a 4-channel amp similar to the APA4320.
Using my Honeytone referencing non-bridging terminals, I sense audio signal on Q24 and Q22 pre-driver MPSA06/56. Each channel has a set similar. All 4 channels I can observe audio on some of the pre-drivers. There is barely any output at speaker terminals however and while my honeymoon can sense something, hooking up a regular 4-ohm speaker doesn't produce any output at all.
Things I've checked.
Amp is gettin getting +-32vRail
Regulators are working +-16.7v
No opamps are producing anything significant at outputs, though some of the outputs have no audio on them. I did change out a few of the OpAmps but that didn't help.
Amp controls seem to be working as I can sense gain changes at the MPSA06/56 at the front of the drive circuits.
Each channel of this amp has quite a few driver transistors all seem to be testing out alright.
Any clues or direction?
Using my Honeytone referencing non-bridging terminals, I sense audio signal on Q24 and Q22 pre-driver MPSA06/56. Each channel has a set similar. All 4 channels I can observe audio on some of the pre-drivers. There is barely any output at speaker terminals however and while my honeymoon can sense something, hooking up a regular 4-ohm speaker doesn't produce any output at all.
Things I've checked.
Amp is gettin getting +-32vRail
Regulators are working +-16.7v
No opamps are producing anything significant at outputs, though some of the outputs have no audio on them. I did change out a few of the OpAmps but that didn't help.
Amp controls seem to be working as I can sense gain changes at the MPSA06/56 at the front of the drive circuits.
Each channel of this amp has quite a few driver transistors all seem to be testing out alright.
Any clues or direction?
Looking at the signal starting at the RCA jacks (using your scope), where do you lose the signal?
What's the DC voltage on VEE+ and VEE-?
What's the DC voltage on VEE+ and VEE-?
All opamps are seeing +-17.4vDC. Rail voltage is +-31.25vDC.
I traced signal with my scope on Channel 1 referencing RCA shield. All 4 channels are doing the same thing. After U8 - Pin1 opAmp, its still a clean sine wave until the other side of R141. From there the signal turns into more of a pyramid than a sine wave. Then all components tested past R141 reveals a square wave that gets reduced further and further out I go through the drive circuitry.
Channel 1. I removed outputs Q43, Q50, and final drivers, Q44 and Q49. Still seeing square wave on everything I've tested after R141.
I traced signal with my scope on Channel 1 referencing RCA shield. All 4 channels are doing the same thing. After U8 - Pin1 opAmp, its still a clean sine wave until the other side of R141. From there the signal turns into more of a pyramid than a sine wave. Then all components tested past R141 reveals a square wave that gets reduced further and further out I go through the drive circuitry.
Channel 1. I removed outputs Q43, Q50, and final drivers, Q44 and Q49. Still seeing square wave on everything I've tested after R141.
I just saw you meant VEE. There is no voltage on those +/- there (Tested at R178 and R183). Tracing around, all 4 channels are connected of course.
U13 LM393N
1: ~11.3v (Drops from about 13 with meter touching it)
2: 5.61
3: 11.80
4: 0
5: 5.58
6: ~11.3v (Drops from about 13 with meter touching it)
7: 0.138
8: 0
1: ~11.3v (Drops from about 13 with meter touching it)
2: 5.61
3: 11.80
4: 0
5: 5.58
6: ~11.3v (Drops from about 13 with meter touching it)
7: 0.138
8: 0
Pin 8 should have 12v.
DCV measured directly across pins 1 and 2 of U6?
DCV measured directly across pins 4 and 5 of U6?
DCV measured directly across pins 1 and 2 of U6?
DCV measured directly across pins 4 and 5 of U6?
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Its got 13.8v now. I found the issue. There was a disconnected trace between R189 and U6 pin 2. The trace was going very close and then under the board between 2 caps and for the life of me I could not get a solid connection between those two points. I installed a trace bridge between the two points.
Amp is drawing about 1.7A of current now, getting warmer. Seems I have a clean signal on speaker terminals now.
I'm going to throw this back into the sync and test it further.
Thank you!
Amp is drawing about 1.7A of current now, getting warmer. Seems I have a clean signal on speaker terminals now.
I'm going to throw this back into the sync and test it further.
Thank you!
So, the mini honeytone was actually detecting a very faint signal. As I mentioned, the sine wave turned into a square wave well after the last op-amp, and each level just got softer and softer. To me the mini amp actually sounded the kind of OK since its just a simple test tool; square waves sounded kind of similar to sine waves. The amp puts no load down. The scope told a better story.
I feel lucky I was able to find that broken trace. I'll bench test this amp a bit better tomorrow.
I feel lucky I was able to find that broken trace. I'll bench test this amp a bit better tomorrow.